Gases passing up through an accumulation of particles with sufficient velocity induce movement of the particles and bulk circulation of the particles in containment. Particles become levitated by the gases as a result of momentum and energy transfer. A bed of circulating particles that are levitated in a fluid are “fluidized” in that the mixture of solids and gases take on fluid-like properties and characteristics.
A typical fluidized bed is shown in FIG. 1. FIGS. 2-6 illustrate examples of several specific types of fluidized beds, which include an expanded bed 100 (FIG. 1), a slugging bed 200 (FIG. 2), a bubbling bed 300 (FIG. 3), a channeling bed 400 (FIG. 4), and a spouted bed 500 (FIGS. 5 and 6).
Slugging bed 200 (FIG. 2) has bubbles in layer 205 that divide bed 200 into particle containing layers 210. Boiling bed 300 (FIG. 3) has bubbles 305 that are many times larger than the solid particles 310. Channeling bed 400 (FIG. 4) forms channels 405 in bed 400 by gas passing through particles 410 and most of the gas passes through channels 405 rather than particles 410.
Spouting bed 500 (FIGS. 5 and 6) has gas traveling through bed 500 to form a single spout 515 through which some particles 510 are elevated by a central gas column 505 and ejected out of the particle bed to form a fountain of particulates 525, and then fall to outside of spout 505. At higher airflow rates, agitation may become more violent and the movement of solids may become more vigorous.
Spouted bed 500 is one type of fluidized bed commonly found with larger and denser particles. In spouted bed 500, a column 505, which passes through solids 510, forms a chimney 515, which is also referred to as a spout 515, which gases and entrained particles pass through. As the spouted gases with entrained particles erupt through a top surface of the bed at 520, the gases disperse and the particles disengage and fall back to the bed surface 520 forming the “fountain” 525 of particles.
Spouted beds 500 usually include a cylindrical body 527 with a conical bottom portion 530. A straight wall 535 generally forms conical bottom portion 530. Conical bottom portion 530 confines fluidizing gases at an apex 540 and causes the fluidizing gases to have a much higher velocity than elsewhere in the bed 500. This may cause particles to slug as a group in the area of apex 540, just above a gas inlet 545. Such slugging action induces vertical reciprocation in the bed 500 that abrades particles against other particles 510 and the wall 535 of the vessel. A portion of the potential and kinetic energy supplied by gas stream 515 is expended in inducing the reciprocations. This use of the bottom portion 530 having a conical shape can lead to a condition where a “flat” slug forms at the base of the particle emulsion, directly over the gas inlet 545, which lifts the particles until the gas can create a spout along the axis of symmetry. At this point, the bed 500 drops, nearly filling the apex 540 of the cone at the inlet 545. The slugging action moves the particles in the descending emulsion in a reciprocating manner, causing particles to collide and rub against other particles 510 and the vessel wall 535. This action abrades and attrits away the surface portion from the particles 510, which is not desirable in coating and drying applications.
For example, nuclear fuel particle coating is a chemical vapor deposition process that can take place in a spouted bed. The abrasion may cause flattened surfaces and irregular (non-spherical) particle shapes, resulting in localized stress concentrations in the shells of coated nuclear fuel particles that may lead to an increased probability of failure in service. Abrasion of grain, coal particles, and other carbonaceous materials can lead to explosive dust mixtures in air. In combustion processes, however, the abrasion may be favorable because it helps to remove ash from the particles that would otherwise impede transport of oxygen to the fuel surface and transport of combustion gases away from the particle.
In nuclear fuel coating operations, reactive gases spouting into the bed decompose to form a condensing species (e.g., pyrocarbon, silicon carbide, or other ceramic) that coat the particles. A void created by the movement of gases above the gas inlet, together with slugging action, repeatedly exposes the gas inlet and adjacent wall surfaces to hot reactive gases and the condensing species. This leads to the deposition of accretions near and within the inlet port that causes a maldistribution of gases, interferes with discharging coated particles from the coater vessel when the coating process is complete, and will ultimately require the coater vessel to be dismantled for cleaning or replacement. Furthermore, chipped material from the accretions become entrained in the emulsion and may form counterfeit fuel particles or interfere with uniform deposition of the coatings on a particle, thus increasing the particle failure and rejection rates.